The present invention relates generally to a heat transfer device and more specifically to the structure and method of constructing a sintered heat pipe wick with longitudinal grooves adjacent to the vapor space. Heat pipes with longitudinal grooves lining the inside of the casing, in effect making part of the casing act as a wick, have been known previously. In various versions, these devices have been used either with the grooves uncovered or covered with fine mesh screen. Covered grooved casings provided the highest heat transfer rates reported to date. Uncovered grooves in the casing are also used in heat pipes for the thermal control of spacecraft.
In describing grooved structures it is customary to speak of "lands" and grooves or channels. The lands are the material between the grooves or channels. The sides of the lands define the width of the grooves. Thus, the land height is also the groove depth. The prior art consists of grooved structures in which the lands are solid material, integral with the casing wall. The grooves are made by various machining, chemical milling or extrusion processes.
The grooves are generally of rectangular crossection. However, other shapes more complex have been made and tested. Complex groove structures are quite difficult and costly to fabricate, but have certain performance advantages. The capillary pressure providing flow in a grooved casing heat pipe is determined by the groove width, with narrower grooves providing higher pumping pressures. If the groove is of rectangular crossection, a narrow width will produce a high viscous drag as compared with a groove of the same crossectional area have equal depth and width. These complex crossections are used to provide relatively high capillary pressure and relatively low liquid drag.
In addition to their function in defining the capillary pressure and liquid drag, the lands are thought to play two important roles in the thermal performance of a heat pipe. First, the high thermal conductivity of metallic lands provides the major path for heat to flow to the liquid surface in the evaporator and from the liquid surface in the condenser. This aspect of performance is particularly important with non-metallic working fluids which have relatively poor thermal conductivity. Second, it is believed that thin film evaporation and condensation takes place on the tips of the lands. For this action to be effective the liquid must wet the lands well and there must be a continuous layer of fluid connecting the land tips with its reservoir of liquid in the grooves. However, the reliability and continuity of this layer is doubtful and subject to unpredictable variations. The result may be a large variation in the effective area of the evaporator and condenser, and a major variation in heat pipe performance.
Grooved casing heat pipes provide excellent longitudinal passages for liquid flow, but effectively block circumferential flow. Thus, if either the evaporation or condensation processes are circumferentially non-uniform, as is usually the case to some degree, the liquid returning from the condenser is unlikely to be distributed circumferentially in the same manner as the evaporation rate in the evaporator. This unbalance can cause dryout of some grooves while others are carrying excess liquid. A means of circumferential liquid distribution is required. This has previously been accomplished by interconnecting the grooves in the condenser or by covering the grooves with fine pore mesh screen. Both of these methods, however, represent added costs and complexity.
Wicks made from sintered metal powder are also known. However, these are generally simple homogeneous structures of annular crossection.